Balancing hoists have long been known in which a drum has a length of cable wound and unwound thereon as the drum is rotated in either direction to position a load held by the cable. This arrangement has utilized pneumatically operated hoists which use regulated air pressure acting on a piston to cause cable wind up or pay out by rotation of the drum. See U.S. Pat. No. 3,428,298 for a detailed description of this type of hoist. The load can be raised or lowered by the operator by exerting a low level force on the suspended load which increases or decreases the air pressure acting on the piston slightly, which pressure change is made up by a regulator to lower or raise the load accordingly.
The limited stroke of the piston limits the cable travel that can be obtained, and thus electrical motor driven balancer hoist have been developed, as described in U.S. Pat. Nos. 3,921,959 and 4,807,767.
The servo motor typically drives a planetary reduction gear, the output of which drives the cable wind up drum.
Since the cable is elastically stretchable to a significant degree, it has considerable stored energy when heavily loaded.
If the cable breaks, a hazard can be created by whipping of the cable caused by release of the stored energy when the cable breaks or when there is some other failure. Emergency brakes have been employed to prevent rapid unwinding of the cable in this situation.
The mass of the planetary gearing also increases the momentum of the movable components when winding or unwinding is underway. The control of the servo motor is made more complicated by the cable stretch and the momentum of the rotating components, creating complex dynamics, particularly at the high speeds which the electric servo motor drive systems operate.
The cable must always be maintained in tension during raising and lowering operation of the hoist in order to avoid loose turns in the cable windings on the drum leading to tangling of the windings, interfering with later unwinding. Sensors and complicated software are required to insure that this does not occur.
Thus, the use of a chain in balancing hoists would be preferable to eliminate difficulties in winding of a cable and the hazards associated with cable stretching. The use of a chain in a balancer hoist is shown in U.S. Pat. No. 3,921,959. However, the mass of a chain wound on a drum is relatively great, and when combined with the mass of a planetary gear set, this affects the response of an electric motor driven balancer hoist.
In some electric motor driven balancer hoists, load sensors sense a change in the load on the cable or chain to cause the electric motor to drive a drum to raise or lower the cable or chain balance a load in “float” mode.
The weight of an operator's hand can upset the “float” balance, since the load sensor will react to removal of the operator's hand from the handle.
Alternatively, manipulation of a handle or grip connected to the cable causes the motor to selectively drive the motor so as to raise or lower the load at a rate proportional to an up or down force applied by the operator to the grip.
Automatic controls can also execute raising or lowering motions to programmed stops as when repetitive motion cycles occur.
Such self balancing hoists have been mounted on trolleys traversed along an overhead aluminum rail track system. In order to assist movement of the trolleys, pulling on the cable by the operator in a given direction is sensed by a power cable angle sensor and powered driving of the trolley in that direction is created in response to sensing such cable pull. The cable angle sensor would be problematic with a chain, and has other limitations.
Also, trolleys have in the past been driven by friction wheels engaging a smooth surface on the aluminum rail. However, friction wheel slippage can sometimes occur especially under heavy loads, which slippage upsets the accurate functioning of the control system, as a commanded movement of the trolley may not occur if such slippage is encountered. A hoist utilizing a chain wound up on a drum would be especially troublesome.
It may be desirable to alternatively allow a free wheeling manually induced movement of the trolley, which has not heretofore been provided in a powered 21 trolley system.
Another application of pneumatic balancing hoists is the combining of two such hoists to lift a common load by synchronizing the motion of the two cables as described in U.S. Pat. No. 5,593,138. Again, the problems of improper cable winding may encountered with a lift cable and lift travel is limited by the relative short piston strokes as a practical matter.
It is an object of the present invention to provide an electrically powered balancer hoist using a chain which has a minimum mass of the components rotated by the electrical motor to allow the use of a chain while still providing good performance.
It is a further object of the present invention to provide an electric motor drive chain hoist with an automatic float mode as well as manual mode using a handle grip in which the operator's hand on the handle does not affect the float mode.
It is another object of the present invention which incorporates powered, sensor controlled trolley movement which is accurate and more reliable, and selectively allows free wheeling of the trolley.
It is a further object to provide a double hoist system using a servo motor drive and hoist chain lift.